Thermal image producing devices typically use a control system which selectively heats different areas of a screen. U.K. Patent Specification No. 2117957 discloses one such thermal image generating device for use in developing and testing infra-red radiation sensitive electro-optical equipment. This device has a substrate supporting an array of resistor elements which can be selectively heated by the passage of electrical current therethrough. By selectively heating the resistors, the device produces a controllable thermal image made up of pixels which correspond to respective ones of the resistor elements.
At the rear of the substrate, a heat sink is provided to receive and dissipate the heat produced by the resistors. A problem with such a device is that, since the temperatures of the elements are not all the same (and this is necessary so as to produce an image), the heat flux flowing into the heat sink is not spatially uniform. Moreover, since the image varies with time, the heat flux flowing into the heat sink at one point on its surface will also vary with time.
An object of the invention is to provide a thermal image generating device comprising a resistor array from which the produced heat flux is at least more spatially and temporally uniform.
According to one aspect of the present invention, there is provided a thermal image producing device that has an array of first and second resistor elements and electrical current supply means connected to the elements for enabling them to be selectively heated by the passage of electrical current therethrough. The elements are arranged such that heat produced by the first elements is receivable from the device as a controllably variable thermal image while the heat produced by the second elements renders the distribution of the heat flux produced by all the elements substantially uniform over the area of the array.
According to a second aspect of the invention there is provided a thermal image producing device that has a first planar array of resistor elements, electrical current supply means connected to the elements of the first array for enabling them to be selectively heated by the passage of current therethrough and thereby to produce a controllably variable thermal image at one side of the array, a second planar array of resistor elements positioned at the other side of the first array with each element of the second array adjacent a respective corresponding one of the elements of the first array but insulated from said respective corresponding one element of the first array, and from said one side of the first array, by dielectric material; and cooling means for receiving heat from the said other side of the first array and from said second array, the electrical current supply means being further connected to the elements of the second array for enabling them to be selectively heated by the passage of current therethrough to render the distribution of the heat flux produced by all the elements substantially uniform over the area of the arrays.